Place tomato, capsicum and onion on a baking paper lined tray. Lightly spray with oil, sprinkle with rosemary and season to taste. Bake in a preheated oven at 180ºC for 12-15 minutes.
Meanwhile, layer filo pastry in a 33cm x 9cm rectangular fluted tart pan with removable base. Spray each pastry layer with oil. Gently scrunch up edges and prick pastry base with a fork.
Bake in a preheated oven at 200°C for 10 minutes or until golden brown. Flake John West Tuna into chunky pieces and place into pastry case with roasted vegetables, olives and fetta cheese. Serve immediately.